In the early 1990s, conservationist and TV personality Steve Irwin and his father Bob discovered a new species of turtle in the Burdekin River catchment, in northern Queensland.
Irwin’s turtle, as it was named, is a short-necked, yellowish creature about 30 centimetres in length. It can breathe through its cloaca – its bum – to stay underwater for longer.
This, along with a few other habits, makes it a hard turtle to spot: while Elseya irwini has been recorded in other waters, no one had seen it in the lower Burdekin River for 25 years.
Now, thanks to the still-young method of eDNA sampling, researchers have been able to confirm that Irwin’s turtle is still hanging out below the Burdekin Falls Dam, which opened in 1987.
“Until this rediscovery, we didn’t have any formal records to prove that the Irwin’s turtle was still living in the lower Burdekin River, and that river has changed a lot since the construction of the Burdekin Falls Dam,” says Professor Damien Burrows, director of the Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER) at James Cook University, and co-author on a paper describing the research, published in BMC Ecology and Evolution.
“It’s reassuring to know they are still living there.”
Environmental DNA analysis, or eDNA analysis, is the process of looking for DNA fragments from a species in a given environment – often water.
The researchers collected water samples across the Burdekin, Bowen and Broken River catchments and tributaries, analysing each sample for fragments of turtle DNA.
“We sampled 37 sites throughout the catchment for the current survey and got positive detections at 18 of those,” says Burrows.
“eDNA may not be evenly distributed throughout a given waterbody, so at each site we take five water samples, each from a slightly different part of the waterbody or site.”
While they haven’t seen the turtle in person in much of its historical range, this DNA shows evidence that it’s there.
“Previously, it’s been very difficult to sample for the Irwin’s turtle because they only live in places where there are crocodiles, or in upland tributaries which are very hard to access,” says Burrows.
“They also don’t come into traps easily and the water they are living in in the Burdekin isn’t clear so you can’t put in underwater cameras to see them.
“But now with eDNA, all we had to do was take a water sample and analyse for their DNA.”
Unfortunately, eDNA can’t give the researchers a clear sense of the number of species present, or their age.
But lead author Dr Cecilia Villacorta-Rath, a research officer at TropWATER, says that their presence does mean that the Burdekin dam has not damaged turtle populations as much as feared.
“We don’t know anything about the demographics of this population, but the fact we have found their eDNA now, despite the dam being built in the 1980s, could point to adult Irwin’s turtles being able to survive in these more turbid water conditions,” says Villacorta-Rath.
It also has implications for the proposed construction of a dam on Broken River – another Irwin’s turtle hotspot.
“These turtles prefer flowing, well-oxygenated water,” says Burrows.
“The Broken River and its tributaries have permanent water flow and thus provide favourable habitat for the turtle. Impounded water is deep and still, so is not considered good habitat for these turtles.
“To assess the effect of any proposed dam on the turtle, we need to better understand their distribution and abundance throughout their entire range.”
Burrows says that eDNA can help with this.
“Now that we have demonstrated that this technique works and is powerful, we intend using this technique at additional sites in the Broken River catchment in the near future. This information will support communities and government in making decisions around any proposed dam.”